EFFECTS OF A CIRCADIAN MUTATION ON SEASONALITY IN SYRIAN-HAMSTERS (MESOCRICETUS-AURATUS)

In Syrian hamsters, exposure to short photoperiods or constant darknes s induces a decrease in gonadotrophin secretion and gonadal regression . After 10-12 weeks, animals undergo spontaneous gonadal reactivation, gonadotrophin concentrations rise, and in males, testes size increase s and spermatogenesis resumes. The tau mutation shortens the period of circadian wheel-running activity by 4 h in the homozygote. Here, we e xamine the impact of this mutation on the reproductive response to pho toperiod change. Seventeen adult tau mutant and nine adult wild-type m ales were housed in complete darkness for 25 weeks and testes size det ermined at weekly intervals. Gonadal regression and subsequent recrude scence occurred in both groups of animals. Regression occurred more ra pidly in iau mutants, with a nadir significantly earlier than wild-typ es but after a similar number of circadian cycles. Rates of testicular recrudescence were similar in both groups. Our data suggest that an a cceleration of the circadian period increases the rate of reproductive inhibition in animals exposed to inhibitory photoperiods. Once initia ted, the rate of spontaneous reactivation may be independent of the ci rcadian axis.